Have a look at the Gina Ford book of weaning. Whether you like her or not, her book has a really sensible guideline to weaning that fits in well with the routine you already have!
Ultimately you want your LO to have three meals a day by around 1 year (although I think my DS was eating 3 meals by about 7 months, still with milk in the am, afternoon and before bed). Based on her suggestions, I would start introducing solids at the 12pm 'feed' as this will become lunch.
Give half the milk at 12pm to take the edge off his hunger, followed by solids (made up with milk) and then top up with the rest of the milk. Over a few weeks, the amount of milk will have reduced as the solids increase (although the number of oz won't really change as you introduce cheese, yoghurt etc!).
You can then do the same at tea and the same at breakfast.
By about 7-8 months, my LO was having:
7am; 6-8oz milk
8am: breakfast (cereal/porridge + milk + fruit, yoghurt)
11.30am: lunch (protein + carb + veg, fruit, yoghurt)
2.30pm: 6-8oz milk
5pm tea (as lunch)
7pm: 6-8oz milk - BED!
It is really essential that you introduce protein swiftly as stores of iron are pretty much gone BY 6 months, so your LO needs to be eating adequate amounts at 6 months to replenish those stores.
Good luck